1. Field
The invention is in the field of non-thermal plasma (NTP) generation cells and treatment systems such as used for treating emission gases from commercial and industrial processing wherein the gases used for such activity contain odors and/or volatile organic compound contaminants and/or hydrocarbon compounds, some of which are considered to be pollutants, and need to be removed from the gas before release of the gas to the atmosphere, and in purification of gases and in sterilization of surfaces.
2. State of the Art
Odorous compounds, which could be organic or inorganic, herein called odors, and/or volatile organic compound (VOC) contaminants and/or hydrocarbon compounds herein called VOCs, emitted into the environment from a range of sources and processes can fill the air in and about residential neighborhoods. Such odors and/or VOCs can range from mildly offensive to intolerable levels. This is a common problem in areas that are in proximity to such sources. Examples of odorous sources include industries that process organic materials such as those that process and produce food for human consumption and industries that produce animal feed for the pet, fish, poultry and hog industry, and general agricultural applications. Other industries that process organic materials and release odors are those that process animal products including meat processing and rendering plants. Other organic odor sources include composting facilities, sewage treatment centers, garbage transfer stations and other industrial organic processing facilities. Generally, these industrial operations exhaust gases from cooking, grinding, drying, cooling, manufacturing, or reduction processes. These exhausts contain low-level concentrations of amines, aldehydes, fatty acids, and volatile organic compounds (VOCs) inherent in the materials processed and those are driven into the exhausted gas stream by the processing activity. These industries typically have large gas flow volumes, ranging from 1,000 to 250,000 actual cubic feet of gas per minute (ACFM) and above.
Agricultural activities that raise animals for food production, such as hog, poultry and dairy farms also emit strong and offensive odors into the environment from manure and barn ventilation odors and these can release offensive odors in sufficient quantity to fill many square kilometers under certain weather conditions.
Additional sources of environmental emissions exist that expel VOCs from non-organic processing, such as solvent evaporation from painting, cleaning, and other general industrial and commercial activities. Some VOCs may have little or no odor, but are considered atmospheric pollutants and/or carcinogens and need treatment to reduce them to harmless compounds. In the case where odors and VOCs are very potent, even concentrations in the parts per billion ranges can be offensive or exceed environmental emission limits and these also need treatment.
There are various systems designed to oxidize and/or reduce odorous and VOC emissions in commercial and/or industrial process gas that is to be emitted into the environment so that the emitted exhaust gas stream is within environmental regulatory limits. Some of these systems use non-thermal plasma (NTP) which is formed in dielectric barrier discharge (DBD) cells to create a wide range of activated species such as activated or Reactive Oxygen Species (ROS) that are then mixed with the gas to be treated so that the organic compounds that humans normally detect as odor, and/or VOCs, are oxidized and/or reduced, typically to carbon dioxide and water vapor, though other products are possible depending on the chemical characteristics of the pollutants, by the energetic ions in the ROS.
Activated species, as described herein, are chemical entities that are created in useful concentrations by the application of sufficient energy, such as through dielectric barrier discharge, to drive the molecules of interest from the ground state into the active state required, with the ground state being the normal state of these molecules typically at a nominal one-atmosphere pressure and 20 degrees C. (or whatever atmospheric and temperature conditions occur at the place of the odor, VOC, and/or organic compound emissions). Activated species are typically designated in literature by “.” as in O. for active oxygen (atomic oxygen in this case). Activation occurs through a number of mechanisms including direct electron collisions or secondary collisions, light absorption, molecular processes involving ionization, or internal excitation.
Dielectric Barrier Discharge (DBD) technology has been used to create the non-thermal plasma (NTP) that generates the activated species required for the purposes of this invention, and as such technology inherently limits the eV that can be applied to the gasses passing through the barrier, it is mainly the Reactive Oxygen Species (ROS) which include a range of hydroxyl radicals, that are involved in this case, though other electron activity assists in the process. For the activated species generated in the NTP field, those ROS species that have the highest reduction potential (between about 2.4 and 5.2 eV) have the shortest availability with half-life concentrations of less than about 100 milliseconds. These react with the odorous molecules that need high reduction potential oxidizers for decomposition. These high reduction potential radicals, and the reactions between these particles and the odorous molecules reacting with them, occur only in the NTP field, as these radicals quickly decay to less active species outside the NTP field. These radicals react with the odorous molecules by oxidation and reduction transformations so that the odorous molecules are transformed to simpler molecular forms that are no longer detectable as odor. Additional activity occurring within the NTP is that of electron collisions, bombardment and direct ionization, which acts on all molecules within the field, including the compounds of concern. This electron action, as well as creating the ROS of interest, also results in the disruption of the molecular bonds of the odor and/or VOC compounds, which also aids in the ROS activity of oxidation and/or reduction of the odor and/or VOC compounds. The NTP field also creates, within the ROS, a range of lower reduction potential radicals (between about 1.4 and 2.4 eV), and these are longer lived with half-lives from about 100 milliseconds to several minutes. These radicals react with the odorous molecules that respond to this level of reduction potential and oxidation for decomposition. These reactions occur both in the NTP field and in the air stream outside the NTP field, as those radicals are active longer and are carried outside the NTP field by the airflow through the DBD. These longer-lived radicals also effect their changes on the odorous and/or VOC compounds by oxidation and reduction transformations, so that the compounds of concern are transformed to simpler molecular forms that are no longer detectable as odor. Such transformations also ultimately convert the complex organic molecules and hydrocarbon molecules into the most simplified oxides, such as carbon dioxide, hydrogen dioxide (water), nitrogen (N2) and other simplified oxide forms of the elements that were in the original complex compounds.
Four oxidation states of molecular dioxygen are known: [O2]n, where n=0, +1, −1, and −2, respectively, for dioxygen, dioxygen cation, superoxide anion, and peroxide dianion (symbolically expressed as 3O2, 3O2.+, 3O2.−, and 3O2−2). In addition, 3O2, is in a “ground” (not energetically excited) state. It is a free “diradical” having two unpaired electrons. The two outermost pair of electrons in oxygen have parallel spins indicating the “triplet” state (the preceding superscript “3”, is usually omitted for simplicity). Oxygen itself is a common terminal electron acceptor in biochemical processes. It is not particularly reactive, and by itself does not cause much oxidative damage to biological systems. It is a precursor, however, to other oxygen species that can be toxic, including: superoxide anion radical, hydroxyl radical, peroxy radical, alkoxy radical, and hydrogen peroxide. Other highly reactive molecules include singlet oxygen, 1O, and ozone, O3.
Ordinary oxygen does not react well with most molecules, but it can be “activated” by the addition of energy (naturally or artificially derived; electrical, thermal, photochemical or nuclear), and transformed into reactive oxygen species (ROS). Transformation of oxygen into a reactive state from the addition of a single electron is called reduction (Eqn. 1). The donor molecule that gave up the electron is oxidized. The result of this monovalent reduction of triplet oxygen is superoxide, O2.−. It is both a radical (., dot sign) and an anion (charge of −1). Other reactive oxygen species known to be created with NTP, are noted below: (On the Ionization of Air for Removal of Noxious Effluvia [Air Ionization of Indoor Environments for Control of Volatile and Particulate Contaminants with Nonthermal Plasmas Generated by Dielectric-Barrier Discharge] Dr. Stacy L. Daniels, IEEE Transactions on Plasma Science, Vol. 30, No. 4, August 2002):O2+e→O2.−  (Eqn 1)2O2.−+2H+→H2O2+O2.   (Eqn 2)O2.−+H2O2→O2+OH.+OH−  (Eqn 3)O2.−+2H2O→O2+HO2.−+OH.−  (Eqn 4)2O2.−+O2+H2O→2O2+OH−+OH.   (Eqn 5).
For any given reactive oxygen species (ROS), there exists some confirmed or postulated reaction scheme for inter conversion to any of the other species. In any event, several of the above reactive oxygen species may be generated in the NTP and react with odorous molecules to transform them into simpler molecules that are no longer detected as odorous.
It has also been found that non-thermal plasmas from NTP generation cells can be used to purify gases and sterilize surfaces. The NTP and ROS will destroy airborne and surface microbes such as bacteria, molds, yeasts, and viruses. Gases passing through the NTP generation cells are purified and items to be sterilized can be placed in the NTP generations cells or the gases from the NTP generation cells can be circulated around the items to be sterilized to sterilize the items. For example, a gas such as air, nitrogen, or argon can be circulated and recirculated through the NTP generation cell and circulated and recirculated around an item to be sterilized.
Commercial and industrial volumes of contaminated gases to be treated normally have contaminants such as condensing water or other vapors and liquids, particles of some kind, or mixtures of both condensing fluids and particles. A problem arising from the use of dielectric bather discharge (DBD) cells, generating the NTP for treating industrial scale flows of contaminated gases, is that after a period of use, sometimes only a matter of minutes, the contaminants inherent in these gases build up in the cells and cause electrical short circuits in the cells from hot electrodes, across the insulation and support frames, to the ground electrodes. Of course, this interferes with the designed electrical properties of the DBD cell and immediately destroys any ability for the DBD cell to generate the NTP. In this case, it is very likely DBD cell component damage has occurred as electrical arcs have very high temperatures and parts are usually damaged that have been in contact with the arc, and at the very least, cleaning of the DBD cell is necessary to restore the electrical dielectric integrity of the DBD cell, and damaged parts must be replaced.
Parent application Ser. No. 10/628,686, now U.S. Pat. No. 6,991,768, and Ser. No. 11/345,033, now U.S. Pat. No. 7,767,167, disclose hermetically sealing at least the positive or hot electrodes of a DBD NTP generation cell to reduce the short outs and damage to the electrodes in the cell and prolong the effective life of such cells. While the hermetic sealing of at least the portions of at least the hot electrodes in the cell where the contaminated gases to be treated pass over or along such electrodes disclosed in these referenced applications and patents are generally effective in preventing shorts due to build up of contaminants in the cells causing shorting between the electrodes, and is thus generally effective in extending the life of the electrodes and the NTP generating cells and extending the operation time of the cells, it has been found that still, when DBD electrodes are operating, air stream contaminants and plasma field induced chemical reaction by-product compounds can and do adhere to the surface the high voltage and ground electrodes. As deposits on the electrodes accumulate, the glow discharge (millions of random tiny micro discharges) gradually changes to a much smaller number of concentrated, stationary point discharges. During prolonged system operation the glow discharge gradually changes to a much smaller number of point discharges. When this occurs, the volume of desirable plasma induced chemical reactions decreases by about 50% or more, dramatically reducing the destruction efficiency of the targeted Volatile Organic Compounds (VOC) compounds that the system is designed to oxidize.
Depending upon the concentration and the unique mixture of chemical compounds in the contaminated air stream being treated by the NTP system, DBD cell electrode cleaning can be required after about 200 to 500 hours of operation. While this is a great improvement over cells where electrodes are not hermetically sealed, the required manual cleaning of the electrodes is still a time consuming and labor intensive process. Further, it requires significant system down time. This is of particular importance because in many cases NTP systems are used to treat exhaust gases from processes that operate continuously, with high costs associated with stopping plant operation.
Additionally, over the last five to eight years many academic researchers have published papers on the use of “in-plasma” catalysts. This work has demonstrated that combining photocatalysts with non-thermal plasma can increase VOC destruction, alter the type and concentration of NTP by-products and reduce the energy required to achieve target VOC destruction levels. In many instances, destruction efficiencies are improved by 20 to 50+% by adding in-plasma catalysts. Unfortunately, it has been found that in full scale industrial applications of NTP systems, in-plasma catalytic material surfaces become coated by NTP break down products. In relatively short periods of time catalytic activity is significantly or totally impaired. In the case of titanium dioxide as an in-plasma catalyst, the electrodes must be cleaned and the nano-particle titanium dioxide coating must be re-applied frequently. However, again, if this work is completed manually, significant labor and system downtime costs are incurred.
Room still exists for improving the efficiency of NTP generating cells.